Method and apparatus for imparting a scanning movement to a beam of charged particles



. NYGARD METHOD AND APPARATUS FOR IMPARTING A SCANNING mm, m, 1957 J. c.

MOVEMENT TO A BEAM OF CHARGED PARTICLES Filed Sept. 29, 1953 3Sheets-Sheet l Wm a J M m N w mm NYGARD 3 Sheets-Sheet 2 Dem-'10, 1957c,

- METHOD AND APPARATUS FOR IMPARTING A scANNINc MOVEMENT To A BEAM 0FCHARGED PARTICLES Flled Sept 29 1953 8\ 1 1 3 w .w (it J a M w FIG. 3C

Dec; 16, 1957 J. c. NYGARD 9 3 METHOD AND APPARATUS FOR IMPARTING ASCANNING MOVEMENT TO A BEAM 0F CHARGED PARTICLES Filed Sept. 29, 1953 3Sheets-Sheet 3 HG. Q

v' i f l 14/ -15 L Q was I\ 'I\ v v'=o J K l l I P I l INVENTOR jobn-CMald BY W17 C. ATTY DIETHOD AND APPARATUS FOR IIVIPARTIN G A SCANNDIGMOVEMENT TO A BEAM F CHARGED PARTICLES John C. Nygard, Waltham, Mass.,assignor to High Voltage Engineering Corporation, Cambridge, Mass, acorporation of Massachusetts Application September 29, 1953, Serial No.382,923

8 Claims. (Cl. 250-43) This invention relates to a simple method andapparatus for imparting a scanning movement to a beam of chargedparticles, and in particular to a method and apparatus for imparting asubstantially linear scanning movement to a beam of charged particleswithout the use. of complicated electronic devices by allowing theutilized beam current to charge a capacitor and discharge periodicallythrough a discharge device, and applying the resulting wave form to asuitable pair of electrostatic deflector electrodes.

In order to illustrate my invention, I have shown in the accompanyingdrawings apparatus for irradiating a product, material or substance withhigh-energy charged particles, because in such apparatus a substantiallylinear scanning movement of the beam of charged particles isparticularly desirable in order that uniform irradiation may beachieved. However, my invention is not limited to the particularapparatus shown, but my invention may be used to advantage in many othercases where it is desired to impart a substantially linear scanningmovement to a beam of charged particles; that is, where it is desired tocause a beam of charged particles to scan a target area withsubstantially constant linear velocity.

In the drawings:

Fig. 1 is a diagram, partly in longitudinal section and partly inperspective view, illustrating one embodiment of apparatus forpractising the method of my invention;

Fig. 2 is a diagram, mainly in longitudinal section, illustratinganother embodiment of apparatus for practising the method of myinvention;

Figs. 3A, 3B and 3C are diagrams illustrating in detail the operation ofthe apparatus of Fig. 1;

Fig. 4 is a graph illustrating the variation of the deflecting voltagewith time; and

Fig. 5 is a diagram illustrating certain dimensions of the deflectingelectrodes.

Referring to the drawings and first to Fig. '1 thereof, therein is shownone embodiment of apparatus for irradiating a product, material orsubstance with a beam of electrons to which a substantially linearscanning movement is imparted in accordance with my invention. A beam 1of electrons is created by any suitable means, such as an accelerationtube 2 of the type disclosed in U. S. Patent No. 2,517,260 to Van deGraafl and Buechner. Electrons emitted by a source 3 are accelerateddown the tube 2 in a manner not necessary to set forth herein in detail,and enter a tube extension 4 as a concentrated beam. In accordance withmy invention and by means of apparatus to be described in detailhereinafter, a scanning movement is imparted to the beam 1 as it travelsthrough the tube extension 4, so that the beam is fanned out in theplane of the drawing as indicated by the lines 1a and 1b in Fig. 1. Thetube extension 4 terminates in an electron window 5, comprising, forexample, a thin aluminum foil, through which the fanned electron beam10, 1b issues.

A product, material or substance 6 which is to be irradiated issupported in the path of the fanned electron beam 1a, 1b a shortdistance below the electron window 5 by a support 7 which may bestationary or which, as shown in Fig. 1, may be a movable conveyor suchas a belt which travels in a direction perpendicular to the plane inwhich the electron beam is fanned out.

Some of the electrons in the beam 1a, 1b will be reflected by orscattered out of the product 6. Other electrons in the beam may pass bythe product. Most of the electrons in the beam will travel into theproduct, where their energy is expended in ionization and otherprocesses. However, although most of the energy of the electrons in thebeam is thus expended in the product, the entry of such electrons intothe product results in an accumulation of free electric charge in theproduct. For all products except those which are nearly perfectinsulators, this accumulated free electric charge will not remain in theproduct, but will leak off very rapidly to any materials with which theproduct is in contact.

By the term nearly perfect insulator I mean a material which will storeelectric charge within its volume with a very low rate of leakage.Lucite and certain glasses are nearly perfect insulators, but otherwisethere are very few such materials.

By the term utilized beam current I mean the sum of the aforementionedflow of electric charge away from the product: that is, the reflectedand scattered electrons; the electrons which pass by the product; andthe free electric charge which is continuously leaking otf from theproduct.

In accordance with my invention, I accumulate the utilized beam currentin a capacitor and discharge the capacitor periodically to produce avoltage wave-form which is then applied to suitable deflectingelectrodes. To that end, a charge-collector 8 of conductive material issupported on insulating supports 9 so as partially to surround theproduct 6 at the place where it travels through the electron beam 1a,1b. Said charge-collector 8 must be adapted to collect at least aportion of the utilized beam current, and preferably saidcharge-collector is adapted to collect as much of the utilized beamcurrent as possible.

In order that the charge-collector 8 may collect those electrons whichare reflected by or scattered out of the product, or which pass by theproduct, the charge-collector preferably surrounds the product as muchas is convenient without obstructing the path of the primary beam 1a,1b.

In order that the free electric charge in the product may leak off tothe charge-collector 8 and not to ground, the conveyor 7 is preferablyso constructed that its crosssectional resistivity is relatively lowwhile its longitudinal resistivity is relatively high. For example, theconveyor 7 may be composed of insulating material (but not a nearlyperfect insulator) in which strips 7a of conducting material, such asmetallic staples, have been imbedded at appropriate intervals, in orderto provide a plurality of low-resistance paths through the thickness ofsaid conveyor. A conducting member 10 upon which the conveyor 7 slidesas it travels through the beam 1a, 1b completes the electricalconnection between the product 6 and the charge-collector 8.

In order further to assist in preventing electric charge from flowingfrom the product 6 along the conveyor 7 to ground, any grounded supportsfor the conveyor, such as that shown at 11 in Fig. 1, should be spaced asubstantial distance from the charge-collector 8.

The charge-collector 8 is electrically connected to ground through acapacitor 12, which serves-to store the utilized beam current collectedby the charge-collector. As electric charge accumulates in the capacitor12, the voltage drop across it rises, so that the potential of thecharge-collector 8 is raised. A discharge device 13 connected inparallel with the capacitor 12 discharges" the latter whenever thevoltage drop across it exceeds a predetermined value. Said dischargedevice 13 may comprise a simple spark-gap, as shown in Fig. l, or,alternatively, a thyr'atron may be used, as shown at 13a in Fig. 2. Theresultant variation of the voltage across the capacitor 12 is employedto energize a beam-deflecting electric field, as will now be described.

A scanning movement is imparted to the beam 1 within the tube extension4 by a varying electric field between a pair of deflecting electrodes14, 15 which are preferably supported within the tube extension 4 byinsulators 16. One electrode 14 is maintained at constant potential,which may be ground or, as indicated in Fig. l, a Constant biaspotential may be applied thereto by a bias supply 17. The otherelectrode 15 is electrically connected directly to the charge-collector8 by a lead 18, so that the potential of the electrode 15, and hence theintensity of the ele'ctricfield between the deflecting electrodes, isdetermined by the voltage drop across the capacitor 12.

In general, my invention will be most useful in the irradiation ofsubstances by electrons. However, the principle of my invention isequally applicable to beams of positive ions. If. the beam 1 consists ofpositive ions, the tube extension 4 may terminate in an unclosed exitopening of the type disclosed in U. S. Patent No. 2,646,948 to Burrill;or, alternatively, the modifications shown in Fig. 2 may be used. Insaid Fig. 2, the charge-collector comprises a chamber 8a which ishermetically sealed to the tube extension 4 but insulated therefrom byan insulating ring 19; and the chamber 8a itself may conveniently serveas the support for the product, material or substance 6 to be bombardedby positive ions.

The operation of the device is illustrated in Figs. 3A, 3B and 3C. InFig. 3A, the spark gap 13 has just discharged, so that the electrode 15is at ground potential and the condenser 12 is uncharged. The electrode14 is maintained at a fixed potential of V volts by the bias supply 17',so that there is an electric field of V/d volts/cm. between theelectrodes 14, 15; where d is the distance in cm. between the electrodes14, 15. The polarity of V should be the same as that of the chargedparticles in the beam 1, so that the beam 1 is deflected away from thebiased electrode 14, as shown in Fig. 3A. For example, if the beam 1consists of high-energy electrons, V may be fixed at 50 kv.

The beam 1 delivers electric charge to the chargecollector 8 at a rate iamperes, where i is the utilized beam current; and this chargeaccumulates in the capacitor 12, thereby raising the potential V of theelectrode 15 at a rate where C is the capacitance in farads of thecapacitor 12. If the utilized beam current i is constant, as isgenerally the case, V'=it/C, and the potential V of the electrode 15increases lineally in time. When the potential V of the electrode 15equals V, there is no electric field between the electrodes 14, 15, andthe beam 1 is undeflected,

as shown in Fig. 3B.

If the spark gap 13 is adjusted to discharge when the voltage dropacross it is 2V, then the voltage V of the electrode 15 will continue torise until it reaches 2V. At this maximum value 2V the electric fieldbetween the electrodes 14, 15 is such as to deflect the beam 1 towardsthe biased electrode 14, as shown in Fig. 3C. Then the spark gap 13discharges the capacitor 12 and the cycle is repeated. 7

The variation of the voltage V of the electrode 15 with time isillustrated by the graph of Fig. 4. As the capacitor 12 is charged up,the voltage V rises lineally with time t at a rate -i/C. When the sparkgap 13 discharges the capacitor 12, the voltage V drops rapidly to zero.

'4 The frequency of oscillation is the inverse of the time required forthe voltage V to rise to its maximum value of 2V. Hence,

f=i/2VC C. P. S.

bias voltage V in accordance with the relation VL tan A-- where L is thelength of the electrodes, 1! is the distance (measured in the same unitsas L) between the electrodes.

.E is the potential drop through which the particles have beenaccelerated by the acceleration tube (measured in the same units as V),and /c is a number which varies slightly depending upon the velocity ofthe particles. k is never less than 1 nor more than 2, and is generallyapproximately 2, except for very light particles at high energy.Consequently, the maximum angular deflection is independent of theamount of charge on the particles in the beam, and is only slightlydependent upon the mass of the particles in the beam.

It may be noted that when the electrode 15 is at its maximum voltage of2V, the potential drop through which the particles are accelerated isreduced by 2V, since the charge-collector 8 is directly connected to theelectrode 15. Consequently, the potential drop through which theparticles are accelerated varies between E and E2V, so that the energyof the particles varies by a fractional amount ZV/E, which isproportional to tan A.

For 2 MeV electrons and a total deflection of 16, k is 1.21, A is 8, andtan A is 0.1405. If d is 4 cm. and V is -50 kv., L is 27.2 cm. Thepercentage variation of the energy of the electrons in the beam is 5%.

Having thus described the method of my invention together with severalillustrative embodiments of apparatus for practicing the method, it isto be understood that although specific terms are employed, they areused in a generic and descriptive sense and not for purposes oflimitation, the scope of the invention being set forth in the followingclaims.

I claim:

1. That method of imparting a scanning movement to a beam of chargedparticles which method comprises creating and directing a beam ofcharged particles; collecting substantially all of the current of saidbeam; storing the electric charge thus collected; dissipating the storedelectric charge periodically; and subjecting said beam to the deflectingaction of the electric field associated with said stored charge.

2. Apparatus for imparting a scanning movement to a beam of chargedparticles comprising in combination means for creating a beam of chargedparticles; a pair of deflecting electrodes supported so that said beampasses therebetween; a charge-collector for collecting substantially allof the current of said beam; a capacitor electrically connected to saidcharge-collector, whereby the electric charge collected by saidcharge-collector is stored in said capacitor; a discharge device adaptedto discharge said capacitor whenever the voltage drop across saidcapacitor reaches a predetermined value; and means for applying thevariation in the voltage drop across said capacitor to one of saidelectrodes.

3. Apparatus for imparting a scanning movement to a beam of chargedparticles comprising in combination means for creating a beam of chargedparticles; a pair of deflecting electrodes supported so that said beampasses therebetween; a charge-collector for collecting substantially allof the current of said beam; a direct electrical connection between oneof said electrodes and said chargecollector; a capacitor electricallyconnected between said charge-collector and ground; and a dischargedevice adapted to discharge said capacitor whenever the voltage dropacross said capacitor reaches a predetermined value.

4. Apparatus for imparting a scanning movement to a beam of chargedparticles comprising in combination means for creating a beam of chargedparticles; a pair of deflecting electrodes supported so that said beampasses thereoetween; a charge-collector for collecting substantially allof the current of said beam; a direct electrical connection between oneof said electrodes and said chargecollector; means *for maintaining theother of said electrodes at a fixed potential; a capacitor electricallyconnected between said charge-collector and ground; and a dischargedevice adapted to discharge said capacitor whenever the voltage dropacross said capacitor reaches a predetermined value.

5. Apparatus in accordance with claim 4, wherein said discharge devicecomprises a spark gap.

6. Apparatus in accordance with claim 4, wherein said discharge devicecomprises a thyratron.

7. Apparatus for imparting a scanning movement to a beam of chargedparticles comprising in combination an evacuated acceleration tubehaving a source of charged particles at one extremity thereof and a tubeextension at the opposite extremity thereof; a pair of electrostaticdeflector electrodes supported within said tube extension; a capacitorelectrically connected to one of said electrodes;

means for delivering substantially all of the current of said beam tosaid capacitor for the purpose of charging said capacitor; and means forperiodically discharging said capacitor.

8. Apparatus for imparting a scanning movement to a beam of chargedparticles comprising in combination means for creating a beam of chargedparticles; a pair of deflecting electrodes supported so that said beampasses therebe'tween; a charge-collector for collecting substantiallyall of the current of said beam; a direct electrical connection betweenone of said electrodes and said chargecollector; and an electricalconnection between said electrodes including a capacitor and a dischargedevice adapted to discharge said capacitor whenever the voltage dropacross said capacitor reaches a predetermined value, the total currentin parallel with said capacitor being negligible except during dischargeof said capacitor by said discharge device.

References Cited in the file of this patent UNITED STATES PATENTS2,185,135 Schlesinger Dec. 26, 1939 2,385,563 Beers Sept. 25, 19452,429,217 Brasch Oct. 21, 1947 2,456,909 Brasch Dec. 21, 1948 2,561,057Jonker et al. July 17, 1951 2,591,981 Van Overbeek et a1. Apr. 8, 19522,594,513 Stocker Apr. 29, 1952 2,602,751 Robinson July 8, 19522,644,909 De Beurs July 7, 1953

